Polymeric film laminates which include a gas impervious layer are quite suitable packaging materials. Laminates of saran and polyolefins are among the better known of these laminates. One particularly useful prior art laminate of this type is described in U.S. Pat. No. 3,741,253 which issued to Harri J. Brax et al. on June 26, 1973. In the Brax patent, an ethylene vinyl acetate copolymer is the substrate material and the substrate is cross-linked by electron irradiation. Following the cross-linking procedure a saran layer and an additional ethylene vinyl acetate copolymer layer are coated by extrusion onto the substrate. However, neither of these two subsequent layers are cross-linked.
In another prior art patent, U.S. Pat. No. 3,821,182 which issued to William G. Baird, Jr. et al on June 28, 1974, a polyethylene/saran/polyethylene laminate is irradiated by electrons prior to being stretched and, as a result of multiple passes through the electron beam, all layers receive approximately equivalent exposure to the radiation and layers of the same material are cross-linked by the radiation to the same extent.
It has now been surprisingly discovered that certain laminate structures are improved and may be successfully stretched and oriented when all layers have been irradiated and like layers have not been cross-linked to the same extent. Thus, it is one object of the present invention to provide a process for producing laminates in which all layers have been irradiated and cross-linked but in which at least one layer is cross-linked to a greater extent than any other layer.
In addition, in certain prior art laminates such as polyolefin/saran laminates, dimensional stability and delamination have been problems for those structures subjected to severe abuse, particularly at elevated temperatures. Accordingly, it is another object of the present invention to provide a laminate material having superior abuse resistance and dimensional stability.
The foregoing and other objects may be achieved by an irradiated laminate wherein one of the layers has been irradiated to a greater extent than any other layer. This discovery will be better understood by reference to the paragraphs below.